How do we get username printed in the email notification body from logged in user who made form submission?

What I'm trying to achieve; the user who is logged in and submitted the form; I want to include his username in the message body.

Additionally, how do I change form submission email subject line which seems to be hard coded already?

Existing code: block_express_form_submission.php

<?php

defined('C5_EXECUTE') or die("Access Denied.");

$submittedData = '';
foreach ($questionAnswerPairs as $questionAnswerPair) {
$submittedData .= $questionAnswerPair['question']."\r\n".$questionAnswerPair['answerDisplay']."\r\n"."\r\n";
}
$formDisplayUrl = URL::to('/dashboard/reports/forms') . '?qsid='.$questionSetId;

$body = t("
There has been a submission

%s

To view all of this form's submissions, visit %s

",$submittedData, $formDisplayUrl);